Thermal Decomposition Mechanism for Ethanethiol † † † ‡ † † Angayle K

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Thermal Decomposition Mechanism for Ethanethiol † † † ‡ † † AnGayle K. Vasiliou,*, Daniel E. Anderson, Thomas W. Cowell, Jessica Kong, , William F. Melhado, † † Margaret D. Phillips, and Jared C. Whitman † Department of Chemistry and Biochemistry, Middlebury College, Middlebury, Vermont 05753, United States

ABSTRACT: The thermal decomposition of ethanethiol was studied using a 1 mm × 2 cm pulsed Δ → silicon carbide microtubular reactor, CH3CH2SH + Products. Unlike previous studies these experiments were able to identify the initial ethanethiol decomposition products. Ethanethiol was entrained in either an Ar or a He carrier gas, passed through a heated (300−1700 K) SiC microtubular reactor (roughly ≤100 μs residence time) and exited into a vacuum chamber. Within one reactor diameter the gas cools to less than 50 K rotationally, and all reactions cease. The resultant molecular beam was probed by photoionization mass spectroscopy and IR spectroscopy. → Ethanethiol was found to undergo unimolecular decomposition by three pathways: CH3CH2SH   ﬁ (1) CH3CH2 + SH, (2) CH3 +H2C S, and (3) H2C CH2 +H2S. The experimental ndings are in good agreement with electronic structure calculations.

1. INTRODUCTION reaction pathway in which the C−S bond is cleaved. However, Raw energy sources including coal, petroleum, and biomass Baldrige et al. predicts that at higher temperatures (above 1000 K) the molecular elimination channels will be competitive with contain varying quantities of sulfur contaminants. Converting radical processes.10 raw energy sources into usable liquid fuels is a complicated The thermal pyrolysis of ethanethiol shown in (1) has been process involving many steps, but two aspects are common to all studied in static reactors, flow reactors, and flames over the last methods: (1) the use of heat to break chemical bonds of larger − 80 years.1 4 organic molecules into smaller semivolatile or volatile species fi and (2) the need to remove sulfur during the re ning process. CH32 CH SH+Δ→ Products (1) The thermolysis chemistry of the sulfur compounds encountered Our present view on the thermal cracking of ethanethiol, in petroleum and biofuels is poorly understood and, in some 2 cases, completely unknown. The current foundations in this field CH3CH2SH, is based on the early analysis of Sehon et al. who are based on end point chemistry, meaning that the reaction suggested two thermal decomposition channels. The lower temperature pathway (785−938 K) (2) proceeds by molecular mechanisms have been proposed without direct evidence of fi radical intermediates. This knowledge gap hinders any progress rearrangement to form ethylene (C2H4) and hydrogen sul de in refinery cleanup methodology, as current efforts for improving (H2S) sulfur removal technologies are done with an incomplete picture CH32 CH SH→+ C 242 H H S (2) of the molecular level chemistry. − Despite the importance of sulfur in industrial processing, there and, at high temperatures, by a free radical (1005 1102 K) has been very little attention to the pyrolysis of organic sulfur mechanism (3) to form the ethyl radical (C2H5) and sulfanyl (or compounds. The thermal chemistry of ethanethiol has been mercaptal) radical (SH). studied sporadically over the years, which has resulted in several − CH32 CH SH→+ C 25 H SH (3) proposed thermal decomposition mechanisms.1 4 Previous work focused on the disappearance of the parent thiol with little The compounds observed by Sehon et al. were H2S, C2H4,H2, attention, as a result of experimental limitations, to the and CH4. Due to the experimental limitations of Sehon et al. as identification and characterization of thermal decomposition well as other previous studies, reactive intermediates (i.e., SH products. The lack of experimental information on initial radical in (3)) produced as pyrolysis products were unable to be decomposition products has led to disagreement on the reaction detected. For this reason, the current mechanism for the thermal − mechanisms for ethanethiol and many other alkanethiols.5 9 decomposition of ethanethiol is based on end point chemistry Sehon et al.2 and Bamkole et al.5 proposed a radical without direct evidence of radical intermediates. As is the case mechanism for ethanethiol pyrolysis initiated by the cleavage with many molecules including ethanethiol, when end point of the C−S bond. Sehon et al. also suggested a second possible chemistry alone is used to propose thermal decomposition, mechanism along with Thompson et al. that proceeds by mechanisms often the most rich and unexpected chemistry is molecular elimination reactions.8,9 Recent theoretical work by overlooked. Baldridge et al.10 predicts that the molecular elimination channels for alkanethiols are unfavorable at low temperatures Received: March 20, 2017 (below 1000 K) as a result of the relatively high reaction barriers Revised: May 20, 2017 compared with that of the more energetically favored radical Published: May 30, 2017

To understand reaction 1, it is important to examine the thermochemistry of ethanethiol as this will provide a powerful basis for the initial unimolecular decomposition channels. The thermochemistry for ethanethiol is well-known.11 The two − weakest bonds in ethanethiol are the C−S bond,12 14 − ± −1 − DH298K(CH3CH2 SH) = 73.6 0.5 kcal mol , and the C C 13 − ± −1 bond, DH298K(CH3 CH2SH) = 82.5 2.2 kcal mol . On the basis of thermochemistry, it is expected that unimolecular decomposition chemistry will be initiated by the cleavage of these two bonds. In this study, the thermal decomposition of ethanethiol was investigated using a pulsed microtubular reactor and two different methods of detection. The products of the reaction were monitored using 118.2 nm (10.487 eV) vacuum ultraviolet photoionization mass spectrometry (PIMS) and matrix isolation IR spectroscopy. Within the microtubular reactor, ethanethiol seeded in an inert carrier gas is rapidly heated to 1000−1300 K, Figure 1. Schematic of the pulsed microtubular reactor used for and decomposition is initiated. The relatively short residence ethanethiol thermal cracking. Samples were collected in a 5 K cryogenic time in the reactor (50−100 μs) ensures that the observed matrix for analysis by infrared absorption spectroscopy or photoionized chemistry emphasizes the unimolecular processes excluding all with fixed frequency vacuum ultraviolet (VUV) light with ion detection but the most rapid bimolecular chemistry. by a time-of-flight mass spectrometer.

2. EXPERIMENTAL SECTION matrix isolation apparatus is that it allows for accumulation of A high-temperature pulsed microtubular reactor (or hyper- nascent thermal decomposition products over time, promoting thermal nozzle) was used to decompose ethanethiol. The good signal-to-noise, while simultaneously isolating the nascent microtubular reactor is a version of the Chen−Ellison reactor products from each other within the matrix to prevent any that has been used for several years to produce reactive secondary reactions. − intermediates.15 26 The hyperthermal nozzle features a (1 mm The photoionization time-of-flight mass spectrometer that was i.d. × 3 cm long) SiC tube that can be heated up to 1700 K, with used in these experiments has been described in more detail 35,36 the temperature monitored by a type C thermocouple mounted elsewhere; therefore, it is only briefly discussed here. In to the outer wall of the SiC tube. A benefit of the microtubular contrast to the matrix isolation setup, as the nascent cracking reactor is the short residence time (50−100 μs).27 This short products exit the microtubular reactor, the supersonic jet is residence time eliminates problems one might face in a typical skimmed approximately 3−5 mm after exiting the SiC tube and vacuum pyrolysis experiment, including secondary reac- intersects with a 10.487 eV (118.2 nm) laser beam. The 10.49 eV − tions.28 34 light is generated by the frequency tripling of the third harmonic Thermal cracking products are produced by pulsing (356.6 nm, 30 mJ/pulse) of a Nd:YAG. The new ions are injected − fl fl ethanethiol, CH3CH2SH, seeded in an inert gas (roughly 1 2 by a positively biased repeller plate into a re ection time-of- ight atm) through the resistively heated SiC tube into a vacuum tube and accelerated into the drift zone by a strong electric field. chamber. The valve can be fired at a nominal rate of 10−50 Hz. At the end of the flight tube, the ions are reflected back down to The ethanethiol/Ar (≤0.1%) mixture is injected into the SiC the microchannel plate detector biased at negative voltage. The tube where it undergoes thermal decomposition and expands PIMS provides mass-to-charge (m/z) information that comple- supersonically into a vacuum chamber (≤10−6 Torr). The ments the vibrational frequencies obtained via the matrix rotational, vibrational, and translational temperatures drop isolation experiment. The PIMS, unlike the matrix experiment, rapidly within a few reactor diameters. The dynamics of the does not require an accumulation of products for detection, and pyrolysis and gas transport in the reactor has been well therefore, the time duration for an experiment is shorter. A characterized using computational fluid dynamics and will not typical matrix experiment can take up to 4 h for a single be discussed here.27 Two independent spectroscopic techniques, temperature, whereas the PIMS experiments can accomplish matrix isolation IR spectroscopy and PIMS, are used to monitor several temperatures in 1 h. For this reason, the PIMS was used and characterize the output of the microreactor. Figure 1 for quick determination of the optimal temperature for the provides a schematic of the experimental apparatus. thermal decomposition of ethanethiol. Matrix isolation infrared spectroscopy involves freezing the gas that is to be analyzed to a cold CsI window and then probing with 3. RESULTS: THERMAL DECOMPOSITION OF infrared light. The microtubular reactor is mounted to the ETHANETHIOL vacuum shroud of a Sumitumo CKW-21 two-stage closed-cycle Scheme 1 depicts the three product channels for the helium cryostat. The nascent cracking products, seeded in Ar, unimolecular thermal decomposition of ethanethiol found in that exit the reactor pass through a heat shield aperture plate and this work. are deposited on a CsI window cooled to 5 K. The CsI window is Figure 2 shows the PIMS spectra of the decomposition of approximately 2.5 cm from the exit of the reactor. The infrared ethanethiol in the high-temperature microtubular reactor. The spectrum was measured using a Thermo Scientific Nicolet iS50 bottom trace is the mass spectrum that results when ethanethiol FTIR with a mercury/cadmium/telluride (MCT-A) detector. IR seeded in helium (0.067% C2H6S in He) was pulsed through the spectra were collected within the frequency range 4000−500 microtubular reactor at room temperature (300 K). The cm−1. The IR beam passes through a pair of CsI side windows ionization energy (IE) of ethanethiol is 9.2922 ± 0.0007 eV.37 attached to the Sumitumo vacuum shroud. A benefit to the Photoionization with 118.2 nm (10.487 eV) VUV light produced

4954 DOI: 10.1021/acs.jpca.7b02629 J. Phys. Chem. A 2017, 121, 4953−4960 The Journal of Physical Chemistry A Article

Scheme 1. Unimolecular Pathways for the Thermal shows the IR spectrum of ethanethiol at 1300 K, which shows the Decomposition of Ethanethiol production of the SH radical at 2595 cm−1 as a pyrolysis product. The IR band observed in this work compares well with the reported value of 2594 cm−1 in a matrix by Isoniemei et al.45 The small peak at 2605 cm−1 was also observed by Isoniemei et al. and is likely attributed to the fact that the SH radical was undergoing rotation in the matrix. Although the majority of molecules do not rotate in the matrix environment, it is well-known that small + 34 the parent ion, C2H6S ,atm/z 62 and a S1 isotopomer peak at molecules or radicals can undergo nearly unhindered rotation in + 46−49 m/z 64. The (64/62) isotope is calculated to be 4% for C2H6S , the matrix. which is consistent with natural abundance. The PIMS spectra at fi Figure 6 con rms the presence of ethylene, C2H4, as a product 900 and 1100 K were the same as that at room temperature. of ethanethiol decomposition. Figure 6 shows a comparison of When the reactor was heated to 1300 K, product ions at m/z 33 the matrix IR spectrum of CH CH SH heated to 1300 K with and 34 were observed, which arise from the ionization of the SH 3 2 ± 38 ± that of a sample of pure ethylene that was deposited at room radical (IE = 10.4219 0.0004 eV) and H2S (IE = 10.4607 39 temperature. Figure 6 shows distinctive bands for ethylene in the 0.0026 eV). Figure 3 shows the PIMS spectra that result when wavelength ranges 1450−1410 and 1000−900 cm−1. The IR the reactor is heated to 1400 K. The bottom trace in Figure 3 is −1 ν ν bands at 1439 and 947 cm in Figure 6 belong to 12 and 7 of the room-temperature mass spectrum of ethanethiol. When the ν ν ν ethylene. In addition to 12 and 7, vibrational modes 11 = 2995 reactor was heated to 1400 K, product ions at m/z 33 and 34 −1 ν −1 cm and 9 = 3081 cm were observed for ethylene. The IR increase from SH and H2S and new features at m/z 15, 28, and 46 fi are detected. The product ion at m/z 15 arises from the spectra in Figure 7 con rm the presence of H2S as a product from ± 40 ethanethiol decomposition. Figure 7 shows a comparison of the ionization of the methyl radical, CH3 (IE = 9.843 0.002 eV), and at m/z 28 from the ionization of ethylene (IE = 10.5138 ± matrix IR spectrum of CH3CH2SH heated to 1300 K with an 41 authentic room-temperature IR spectra for H2S (green trace). 0.0006 eV). The product ion at m/z 46 arises from the − ± The absorption at 1179 cm 1 in the heated CH CH SH ionization of thioformaldehyde, H2CS (IE = 9.376 0.003 3 2 42 ν eV). Although the literature value for the ionization of ethylene spectrum is unmistakably from the 2 vibrational mode in H2S. ν −1 (C2H2) is greater than our 10.487 eV laser pulse, careful studies Vibrational mode 3 = 2635 cm was also observed for H2S. ffi + of the photoionization e ciency curve show that C2H4 can appear around 10.47 eV.41 4. DISCUSSION. DECOMPOSITION MECHANISM OF The PIMS in Figures 2 and 3 are very informative but only ETHANETHIOL confirm the identity of molecular products by m/z. The matrix IR spectra shown in Figures 4−7 confirm the identity of These experiments have enabled us to identify the initial thermal thioformaldehyde (m/z 46), hydrogen sulfide (m/z 34), sulfanyl decomposition products of ethanethiol. Using the high-temper- radical (m/z 33), and ethylene (m/z 28). Matrix IR spectra were ature microtubular reactor, we have shown that ethanethiol collected for ethanethiol decomposition using the microtubular thermally cracks into the three distinct pathways depicted in fi reactor. In Figures 4−7, the blue trace is a background scan of Ar Scheme 1. To our knowledge, this work is the rst to detect at 300 K and the black trace is a background scan of 0.01% reactive intermediates in the thermal decompositions of mixture of CH CH SH/Ar at 300 K. ethanethiol. The three pathways found experimentally in this 3 2 10 The 1300 K matrix IR spectra shown in Figure 4 affirm the work agree well with the theoretical findings of Baldridge et al. presence of thioformaldehyde, CH2S, with IR transitions at 1055, Additionally, the three decomposition channels proposed here 988, and 982 cm−1. These bands closely match those previously are analogous to the currently accepted decomposition reported in Ar matrices by Jacox et al.43 and Torres et al.44 An IR mechanisms for ethanol, which has been studied in great detail −1 ν 43,44 50,51 transition at 3025 cm for 1 was also observed. Figure 5 both experimentally and theoretically.

Figure 2. PIMS spectra of the decomposition products of ethanethiol in a high-temperature microtubular reactor.

4955 DOI: 10.1021/acs.jpca.7b02629 J. Phys. Chem. A 2017, 121, 4953−4960 The Journal of Physical Chemistry A Article

Figure 3. PIMS spectra for appearance of decomposition products from ethanethiol as the temperature of the microreactor is increased to 1400 K. The inset provides a more detailed view of the m/z 46 region.

Figure 4. Matrix IR absorption spectra demonstrating the presence of thioformaldehyde (CH2SH) following the pyrolysis of ethanethiol in a pulsed ﬀ reactor (0.01% CH3CH2SH/Ar). The blue trace is that of the bu er gas Ar at 1400 K; the black and red traces are CH3CH2SH at room temperature and ν −1 ν −1 ν −1 1300 K, respectively. The known bands 6 = 982 cm , 4 = 988 cm , and 3 = 1055 cm of CH2SH are assigned.

 ± −1  ± The pathways for formation of CH3CH2 +SH(Scheme 1a) cleavages of C S (73.6 0.5 kcal mol ) and C C (82.5 2.2  −1 and CH3 +H2C S+H(Scheme 1b) are from the bond kcal mol ), respectively. Although it has been shown at a range

4956 DOI: 10.1021/acs.jpca.7b02629 J. Phys. Chem. A 2017, 121, 4953−4960 The Journal of Physical Chemistry A Article  50 atom, H2C CH2 +H. The thioformaldhyde (Scheme 1b) observed in both PIMS and IR experiments results from the  isomerization of the CH2SH radical upon the C C bond →  rupture (CH2SH H2C S + H). This analogous isomerization →  (CH2OH H2C O) is also observed in the thermal cracking of ethanol.50  The pathway for formation of H2C CH2 +H2S(Scheme 1c) proceeds through an intramolecular elimination of ethanethiol.

The barrier of formation for CH2CH2 and H2Sviathe intramolecular pathway (4) is 75.9 kcal/mol, which is ≈2 kcal/ mol higher than the measured CS bond enthalpy.10 The intramolecular pathway is also the most thermodynamically favorable pathway. This unimolecular intramolecular pathway (4) has been observed experimentally and theoretically in ethanol and found to have a similar barrier to decomposition of 75.9 kcal/mol.44 The shock tube and experimental study by Park et al.51 determined this product channel to be the dominant Figure 5. Matrix IR absorption spectra demonstrating the presence of − the sulfanyl radical (SH) resulting from the thermal cracking of channel below 10 atm for ethanol in the temperature range 700 ethanethiol in a pulsed reactor (0.01% CH3CH2SH/Ar). The blue trace 2500 K. A second combined shock tube and theoretical study by is that of the buffer gas Ar at 1400 K; the black and red traces are Sivaramakrishnan et al.50 also found the intramolecular channel CH3CH2SH at room temperature and 1300 K, respectively. The single − to be a primary pathway for the unimolecular decomposition of vibrational mode of SH has been assigned at 2595 cm 1. ethanol. As hydrogen radicals in reactions a and b in Scheme 1 and the of pressures and temperatures within the microtubular reactor methyl radical in the Scheme 1b reaction are produced, that the effective residence time is on the order of tens of discussion must be given to the possibility of bimolecular microseconds, we expect that the lifetime of the product reactions involving these radicals with ethanethiol, specifically CH3CH2 radical is too short for observation and rapidly with reference to the intramolecular pathway. An advantage of decomposes in the SiC reactor into ethylene and a hydrogen the microtubular reactor used in our work is the short residence

 Figure 6. Matrix IR absorption spectra demonstrating the presence of ethylene (H2C CH2) following the thermal cracking of ethanethiol in a pulsed ﬀ reactor (0.01% CH3CH2SH/Ar). The blue trace is that of the bu er gas Ar at 1400 K, the black trace is CH3CH2SH at room temperature, the green trace −1 is a spectrum of pure ethylene at 300 K, and the red trace results from heating CH3CH2SH to 1300 K. The IR features at 1439 and 947 cm in the ν ν spectrum at 1300 K clearly belong to 12 and 7 of ethylene.

4957 DOI: 10.1021/acs.jpca.7b02629 J. Phys. Chem. A 2017, 121, 4953−4960 The Journal of Physical Chemistry A Article

fi Figure 7. Matrix IR absorption spectra demonstrating the presence of hydrogen sul de (H2S) following the pyrolysis of ethanethiol in a pulsed reactor ff (0.01% CH3CH2SH/Ar). The blue trace is that of the bu er gas Ar at 1400 K, the black trace CH3CH2SH at room temperature, the green trace is a −1 spectrum of pure H2S at 300 K, and the red trace is the spectrum resulting from heating CH3CH2SH to 1300 K. The IR absorption at 1179 cm in the fi ν 1300 K spectrum is con dently assigned to 2 of H2S. times, which help to minimize rapid secondary radical reactions. of magnitude faster than the most rapid of the possible It has been shown by both experience and extensive simulation bimolecular reactions of the H atom with ethanethiol (8). that the effective residence time in the microtubular reactor is on Although it is still possible that a small fraction of products − μ 27,52  the order 50 100 s. The possible H radical reactions with H2C CH2 and H2S could result from the bimolecular ethanethiol and respective second-order rate constants reported substitution channel (8), it is much more likely based on by Zhang et al.53 are as follows: energetics and the short residence time of the microtubular reactor that the proposed intramolecular elimination channel (4) H abstraction from CH3  is the major source of H2C CH2 and H2S in this study. CH32 CH SH+→ H CH 22 CH SH + H 2 In addition to the bimolecular channels of CH3CH2SH with H −−−13 3 1 1 atoms, we have also considered the reactions of the methyl k(1400 K)=× 6.31 10 cm molecules s (5) radicals with ethanethiol. There are five likely reactions of the methyl radical with ethanethiol. Three of the five involve a H abstraction from CH2 hydrogen abstraction by the methyl radical from the CH3,CH2, or SH groups of ethanethiol. In each of these abstraction CH32 CH SH+→ H CH 3 CHSH + H 2 reactions methane, CH4, is the product. Although the ionization −−−12 3 1 1 ± 54 k(1400 K)=× 6.35 10 cm molecules s (6) energy of CH4 (IE = 12.618 0.004 eV) is above the 10.487 eV of our VUV laser, it is very straightforward to detect CH4 using IR H abstraction from SH spectroscopy. A pure sample of methane was deposited at room CH CH SH+→ H CH CH S + H temperature in the matrix for comparison with the heated 32 32 2 ethanethiol spectra. Methane was not observed in any of the −−−11 3 1 1 k(1400 K)=× 6.30 10 cm molecules s (7) spectra generated from the thermal cracking of ethanethiol. The fi lack of CH4 in the matrix experiments allows us to con dently substitution channel eliminate all three hydrogen abstraction reactions by the methyl radical. The remaining two potential bimolecular channels CH CH SH+→ H CH CH + H S 32 32 2 involving the methyl radical are a substitution analogous to (8) −−−11 3 1 1 k(1400 K)=× 5.37 10 cm molecules s (8) and a second substitution reaction. In the study by Guan et al.27 the more rapid radical/radical substitution channel → × −10 3 reaction of CH3 + HCO CH4 +CO(k =2 10 cm CH32 CH SH+→−+ CH 3 H 3 C SH CH 32 CH (9) molecules−1 s−1) was studied in the microtubular reactor during the pyrolysis of acetaldehyde. It was determined that if the initial substitution channel reactant concentration was less than 0.1%, then the half-life of CH CH SH+→−−−+ CH H C S CH CH H (10) radicals would be approximately 200 μs between 1200 and 1300 32 3 3 2 3 ff  ± 55 K. Since the e ective reaction residence time is much shorter The products methanethiol, H3C SH (IE = 9.446 0.010), fi − − − than this in the microtubular reactor, it is expected that even at in reaction 9 and methyl ethyl sul de, H3C S CH2 CH3 (IE = elevated temperatures, if the initial concentration of the reactant 8.54 ± 0.1),56 in reaction 10, if present, would have been detected is kept small, secondary reactions are minimized if not excluded in both PIMS and matrix IR experiments. The PIMS spectra of all together. The radical reaction studied in Guan et al. is an order heated ethanethiol revealed no product peaks at m/z 48 or 76,

4958 DOI: 10.1021/acs.jpca.7b02629 J. Phys. Chem. A 2017, 121, 4953−4960 The Journal of Physical Chemistry A Article corresponding to methanethiol and methyl ethyl sulfide, (6) Malisoff, W. M.; Marks, E. M. Thermal Behavior of Sulfur respectively. Additionally, no vibrational modes were observed Compounds in Hydrocarbon Solvents IAliphatic Mercaptans. Ind. for either species in the IR. Specifically, intense vibrational modes Eng. Chem. 1931, 23, 1114−1120. ν −1 ν −1 ν (7)Tsang,W.ThermalDecompositionofSomeTert-Butyl for methanethiol ( 1 = 3015 cm , 6 = 1072 cm , and 8 = 710 −1 57 fi ν −1 ν Compounds at Elevated Temperatures. J. Chem. Phys. 1964, 40, cm ) and methyl ethyl sul de ( 19 = 2976 cm , 22 = 1457 − cm−1, ν = 1446 cm−1, and ν = 1269 cm−1)58 were all absent in 1498 1505. 7 11 (8) Thompson, C. J.; Meyer, R. A.; Ball, J. S. Thermal Decomposition the IR. The lack of spectral signatures in both experimental of Sulfur Compounds. I. 2-Methyl-2-Propanethiol. J. Am. Chem. Soc. techniques is convincing evidence that the substitution reactions 1952, 74, 3284−3287. 9 and 10 are not taking place during the thermal cracking of (9) Thompson, C. J.; Meyer, R. A.; Ball, J. S. Thermal Decomposition ff ethanethiol. All e orts to detect the methyl radical in the matrix of Sulfur Compounds. II. 1-Pentanethiol. J. Am. Chem. Soc. 1952, 74, failed. Previous studies have shown that it is difficult to detect the 3287−3289. methyl radical as a pyrolysis product using matrix isolation (10) Baldridge, K. K.; Gordon, M. S.; Johnson, D. E. Thermal spectroscopy.22,24,59 Decomposition of Methanethiol and Ethanethiol. J. Phys. Chem. 1987, 91, 4145−4155. 5. CONCLUSION (11) Pedley, J. Thermochemical Data and Structures of Organic Compounds; Thermodynamics Research Center (TRC): College We have studied the thermal decomposition of ethanethiol in a Station, TX, 1994; Vol. 1, pp 252−253. pulsed microtubular reactor at short residence times (50−100 μ (12) McCullough, J. P.; Hubbard, W. N.; Frow, F. R.; Hossenlopp, I. s). The reactions were monitored with matrix isolation A.; Waddington, G. Ethanethiol and 2-Thiapropane: Heats of spectroscopy and VUV photoionization mass spectrometry. Formation and Isomerization; The Chemical Thermodynamic Proper- The experimental data collected are explained by the ties from 0 to 1000 K. J. Am. Chem. Soc. 1957, 79, 561−566. unimolecular decomposition of ethanethiol to its pyrolysis (13) Luo, Y.-R. Handbook of Bond Dissociation Energies in Organic products. The thermal decomposition of ethanethiol produces Compounds; CRC Press: Boca Raton, FL, 2002. three unique product channels: (CH3CH2 + SH), (CH3 + (14) Wilson, S. H. S. On the near Ultraviolet Photodissociation of   Hydrogen Sulphide. Mol. Phys. 1996, 88, 841−858. H2C S + H), and (H2C CH2 +H2S). No evidence was found for bimolecular chemistry. (15) Jochnowitz, E. B.; Zhang, X.; Nimlos, M. R.; Varner, M. E.; Stanton, J. F.; Ellison, G. B. Propargyl Radical: Ab Initio Anharmonic Modes and the Polarized Infrared Absorption Spectra of Matrix-Isolated ■ AUTHOR INFORMATION − HCCCH2. J. Phys. Chem. A 2005, 109, 3812 3821. Corresponding Author (16) Kato, S.; Ellison, G. B.; Bierbaum, V. M.; Blanksby, S. J. Base- *E-mail: [email protected]. Phone: 802-443-5517. (A. Induced Decomposition of Alkyl Hydroperoxides in the Gas Phase. Part K. Vasiliou) 3. Kinetics and Dynamics in HO- + CH3OOH, C2H5OOH, and Tert- − ORCID C4H9OOH Reactions. J. Phys. Chem. A 2008, 112, 9516 9525. (17) Ormond, T. K.; Hemberger, P.; Troy, T. P.; Ahmed, M.; Stanton, AnGayle K. Vasiliou: 0000-0001-5963-4014 J. F.; Ellison, G. B. The Ionisation Energy of Cyclopentadienone: A Present Address Photoelectron−photoion Coincidence Study. Mol. Phys. 2015, 113, ‡ Department of Chemistry, University of Washington, Seattle, 2350−2358. WA 98195-1700 United States. (18) Robichaud, D. J.; Scheer, A. M.; Mukarakate, C.; Ormond, T. K.; Notes Buckingham, G. T.; Ellison, G. B.; Nimlos, M. R. Unimolecular Thermal fi Decomposition of Dimethoxybenzenes. J. Chem. Phys. 2014, 140, The authors declare no competing nancial interest. 234302. (19) Scheer, A. M.; Mukarakate, C.; Robichaud, D. J.; Nimlos, M. R.; ■ ACKNOWLEDGMENTS Ellison, G. B. Thermal Decomposition Mechanisms of the Methox- This research was supported by grants provided by the National yphenols: Formation of Phenol, Cyclopentadienone, Vinylacetylene, − Science Foundation (CHE-1566282) and by donors of the and Acetylene. J. Phys. Chem. A 2011, 115, 13381 13389. American Chemical Society Petroleum Research Fund. The (20) Buckingham, G. T.; Ormond, T. K.; Porterfield, J. P.; Hemberger, research reported in this publication was supported by the P.; Kostko, O.; Ahmed, M.; Robichaud, D. J.; Nimlos, M. R.; Daily, J. W.; Institutional Development Award (IDeA) program from the Ellison, G. B. The Thermal Decomposition of the Benzyl Radical in a Heated Micro-Reactor. I. Experimental Findings. J. Chem. Phys. 2015, National Institute of General Medical Sciences of the National 142, 044307. Institutes of Health under grant no. P20GM103449. Its content (21) Vasiliou, A.; Nimlos, M. R.; Daily, J. W.; Ellison, G. B. Thermal is solely the responsibility of the authors and do not necessarily Decomposition of Furan Generates Propargyl Radicals. J. Phys. Chem. A ffi represent the o cial views of NIGMS or NIH. 2009, 113, 8540−8547. (22) Vasiliou, A. K.; Piech, K. M.; Reed, B.; Zhang, X.; Nimlos, M. R.; ■ REFERENCES Ahmed, M.; Golan, A.; Kostko, O.; Osborn, D. L.; David, D. E.; et al. (1) Sabatier, S.; Mailhe, C. R. General Methods for Direct Preparation Thermal Decomposition of CH3CHO Studied by Matrix Infrared of Mercaptans by Catalysis Starting with the Alcohols. C.R. Chim. 1910, Spectroscopy and Photoionization Mass Spectroscopy. J. Chem. Phys. 150, 1217−1221. 2012, 137, 164308. (2) Sehon, A. H.; Darwent, B. deB The Thermal Decomposition of (23) Vasiliou, A. K.; Kim, J. H.; Ormond, T. 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4959 DOI: 10.1021/acs.jpca.7b02629 J. Phys. Chem. A 2017, 121, 4953−4960 The Journal of Physical Chemistry A Article

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4960 DOI: 10.1021/acs.jpca.7b02629 J. Phys. Chem. A 2017, 121, 4953−4960